1. Technical Field
The invention relates to the field of semiconductor technology and, more particularly, to systems and methods relating avalanche photodiode type semiconductor devices.
2. Background Art
Avalanche photodiodes (APDs) are essentially reverse-biased p-n junctions that are operated at voltages close to the breakdown voltage. Photogenerated carriers in the depletion region of the p-n junction travel at their saturated velocities, and if they acquire enough energy from the field during such transit, an ionizing collision with the lattice can occur. In the process, secondary electron-hole pairs are produced, which again drift in opposite directions, together with the primary carrier, and all or some of them may produce new carriers. This impact ionization leads to carrier multiplication and gain.
In Geiger mode operation, for a short period of time, the APD reverse bias is raised above the breakdown voltage. Above breakdown voltage, an unlimited amount of current can flow through the APD if not in some way controlled. To achieve breakdown, carriers must be present in the p-n depletion region. If not, it is possible to operate the APD above breakdown voltage simply because there are no current carriers to multiply and so no avalanche breakdown takes place. However, the APD has dark carriers (non-photo generated carriers) which primarily originate from thermal generation-recombination, tunneling, and quenching-circuit after-pulsing associated with trapping effects due to material defects. Multiplication of these carriers leads to false detection events (dark counts) and is undesirable. In Geiger mode, the APD is operated by exceeding breakdown voltage for such a short period of time that the probability of a dark current carrier being multiplied is small. A circuit quenches the APD current before it becomes catastrophic.
It is known that an important issue with APDs integrated into system arrays and operated in Geiger mode is breakdown voltage non-uniformity. Should APDs with disparate breakdown voltages be included in the same array, the applied voltage pulse that biases the APDs beyond breakdown, will need to surpass the highest breakdown voltage value of all individual APDs. This can dramatically increase the dark-count-rate (number of false detections), as is shown in FIG. 1 for a Si APD, as numerous APDs will be pulsed well above their breakdown voltages. J. Jackson, A. Morrison, D. Phelan, and A. Mathewson, “A Novel Silicon Geiger-Mode Avalanche Photodiode”, Proceedings IEDM, 32-2, 2002. If breakdown voltage uniformity is to be maintained for low dark count rates, “cherry-picking” selection of APDs with closely matched voltage breakdowns needs to be done. Alternatively, to maintain low dark count rates, complex circuitry needs to be incorporated in the APD array in order to bias each APD slightly above its individual voltage breakdown value. Either approach substantially increases APD array system cost.
It is known that Silicon Carbide (SiC) APDs must be operated at a very high gain of >100 to make single photon detection in Geiger mode possible. Premature edge breakdown and the associated increase in dark current, limit SiC APD operation to relatively low gains. It is well known that employing a sidewall bevel angle in a MESA type APD can suppress the onset of edge breakdown to a certain extent. However, for the high gain levels needed for Geiger mode operation, the non-uniform breakdown increases dramatically between the device contact and edges. Two-dimensional raster scan photocurrent measurements of a state of the art APD, at gains of 50, 130, and 1000 are shown in FIGS. 2 to 4, respectively. A. Beck, B. Yang, X. Guo, and J. Campbell, “Edge Breakdown in 4H—SiC Avalanche Photodiodes”, IEEE J. of Quantum Electron., vol. 40, No. 3, pp. 321-324, 2004. At a gain of about 1000, non-uniform breakdown manifests itself at the edge of the device. This non-uniformity is primarily due to field crowding at the device edges and sets an upper limit to the gain attainable by the APD.
It is known that the excessive leakage current and premature breakdown at the APD junction edges can be alleviated by incorporating a plurality of planar guard rings around the periphery of the APD. An example of a planar guard ring structure 42 is shown in prior art FIG. 5. See, U.S. Pat. No. 4,857,982.
It is known that an APD can be front or back side illuminated depending on the specifics of the application and the design.
It is also known that for solar blind applications (wavelength below 280 nm), using 4H—SiC Avalanche Photo Diodes (APDs) hole-initiated impact ionization is necessary to minimize excess noise. X. Guo, L. Rowland, G. Dunne, J. Fronheiser, P. Sandvik, A. Beck, and J. Campbell, “Demonstration of Ultraviolet Separate Absorption and Multiplication 4H—SiC Avalanche Photodiodes”, IEEE Photon. Technol. Lett., vol. 18, pp. 136-138, 2006; and, S. G. Sridhara et al., “Absorption coefficient of 4H silicon carbide from 3900 to 3250 A”, J. Appl. Phys., vol. 84, no. 5, pp. 2963-2694, 1988. This is due to the disparate hole and electron impact ionization coefficients in 4H—SiC, which result in very low k values (k of about 0.02 is the ratio of the electron and hole impact ionization coefficients in SiC) for hole initiated impact ionization.
Prior Art Solar Blind SiC APD Design 1:
Hole-initiated impact ionization is taking place in the structure of FIG. 6. At solar blind UV wavelengths, the photons are predominately absorbed in the 1500 nm n− absorption region. The created electron and hole are accelerated by the field in opposite directions. Only the hole is injected in the high electric-field multiplication region (n+ 120 nm thick) and it's this virtually pure hole-initiated impact ionization that leads to low excess noise. A p-doped SiC substrate overlaid by n-doped epitaxial layers is the preferable structure. However, since commercial p-doped SiC substrates have higher defect densities compared to n-doped substrates, an n-doped substrate with a 2 μm thick p-doped epitaxial layer was used in FIG. 6. X. Guo, L. Rowland, G. Dunne, J. Fronheiser, P. Sandvik, A. Beck, and J. Campbell, “Demonstration of Ultraviolet Separate Absorption and Multiplication 4H—SiC Avalanche Photodiodes”, IEEE Photon. Technol. Lett., vol. 18, pp. 136-138, 2006.
There are two drawbacks to this approach:                1. The p-type metal contacts have to be lateral which creates a very resistive path (through a less than 2 μm corridor), compared to a metal contact at the bottom of the substrate. This increases the APD's RC time constant, which results in higher dark counts (false detections). An RC constant as short as possible is essential for reducing the recovery time for passive quenching and the gate-length for gated passive quenching in Geiger mode operation. A. Beck, G. Karve, S. Wang, J. Ming, X. Guo, and J. Campbell, “Geiger Mode Operation of Ultraviolet 4H—SiC Avalanche Photodiodes”, IEEE Photon. Technol. Lett., vol. 17, pp. 1507-1509, 2005. A high RC time constant also reduces APD bandwidth.        2. n-doped epitaxial layers grown over the heavily doped p+ region, which acts as a substrate, have higher defect densities and thus dark current is increased. The defects trap carriers. When the breakdown voltage is exceeded by a gate pulse in Geiger mode operation, the carriers trapped in defects can be freed and initiate avalanche multiplication, which increases the dark count rate. This is known as afterpusling. J. Jackson, A. Morrison, D. Phelan, and A. Mathewson, “A Novel Silicon Geiger-Mode Avalanche Photodiode”, Proceedings IEDM, 32-2, 2002.        
The structure and p-type lateral metal contacts are shown in FIG. 6. The low excess noise factor for the structure of FIG. 6, for the hole initiated impact ionization at 280 nm is shown in FIG. 7(a). The relatively high dark current, due to the inevitable incorporation of defects when growing epitaxial layers on top of highly doped p− type SiC material is shown in FIG. 7(b).
Prior Art Solar Blind SiC APD Design 2:
In the structure of FIG. 8, epitaxial growth over an n+4H—SiC substrate minimizes dark current due to the commercially available good substrate material quality and the ability to grow low-defect layers on top of n+. The low dark current of this design is shown in FIG. 9. X. Guo, A. Beck, X. Li, and J. Campbell, “Study of reverse dark current in 4H—SiC avalanche photodiodes”, IEEE J. of Quantum Electron., vol. 41, No. 4, pp. 562-567, 2005. In this structure where the photon impinges on p+ material, however, the advantage of low dark current due to epitaxial growth quality is accompanied by the disadvantage of relatively high excess noise due to the mixed electron and hole injection at the >280 nm wavelength of interest, FIG. 10. B. K. Ng et al., “Nonlocal Effects in Thin 4H—SiC UV Avalanche Photodiodes”, IEEE Transactions on Electron Devices, Vol. 50, No. 8, 2003.
For a structure where the photon impinges on p-doped material (similar to FIG. 8), the weakly absorbed 365 nm light gives a mixed carrier initiated multiplication that is close to pure hole multiplication; the excess noise is very low as seen from the measured data at 365 nm, FIG. 10. As the wavelength decreases below 365 nm, light is progressively absorbed at shorter depths and more electrons are injected from the p-doped layers leading to an increase in excess noise. At 230 nm, the majority of the UV light is absorbed in the p-doped layers so the multiplication characteristic corresponds to pure electron multiplication. This leads to high excess noise as seen by the 230 nm symbols in FIG. 10.
While the above cited references introduce and disclose a number of noteworthy advances and technological improvements within the art, none completely fulfills the specific objectives achieved by this invention.